1. Field of the Invention
The present invention is directed toward a process for applying aluminide coatings to superalloy components used in gas turbine applications, and more particularly to an improved vapor phase aluminiding process to coat nickel-based and cobalt-based superalloy components used in the turbine section of gas turbine engines.
2. Discussion of Prior art
Aluminide coatings are applied to provide protection of superalloy turbine components from gas turbine engines to protect the substrate material by many different processes. One such process is set forth in U.S. Pat. No. 3,837,901, ('901) now expired, to Seybolt, incorporated herein by reference and assigned to the assignee of the present invention. In the 901' patent, an aluminide coating is applied by embedding the turbine components in a bed of powders having aluminum as a source. Generally, the active powders set forth in this patent were iron-aluminum compounds mixed with inert alumina, and the powders were activated by passing a halide gas through the carrier while heating the bed to a temperature in the range of about 1650.degree. F. to about 2000.degree. F. Subsequent improvements in the process have included modifications in the aluminum powder sources, modifications in the powder sizes and improvements in the gas delivery systems. These processes have generally come to be known as "pack processes" or vapor phase aluminide processes. These processes have several infirmities. First, since they involve producing coatings requiring specific compositions, they must be carefully mixed so that the coating compositions can be attained. However, once depleted, the mixed metal powders are not readily recyclable, cannot be replenished and present a disposal problem. A second problem associated with these pack processes is that the measuring and moving as well as disposing of the powders is labor intensive. Third, the process, while producing a good protective coating, yields a coating of varying thickness that is not easily controlled. Finally, as temperature demands of gas turbines have increased, cooling passages have been added to the turbine components. The powders of the pack processes frequently clog these channels, and the removal of these powders from these fine cooling passages is a further problem.
What is desired is a coating method that produces a good quality aluminide coating while avoiding the problems associated with the prior art pack processes.